U.S. Pat. No. 3,325,299 (Araujo) discloses phototropic (now known as photochromic) glasses in which copper and/or cadmium halide crystals are precipitated to impart photochromic behavior. The disclosed glasses have R.sub.2 O-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -SiO.sub.2 base compositions. Suitable base glass compositions are disclosed as consisting essentially of, in weight percent on an oxide basis, 40-75% SiO.sub.2, 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3 and at least one alkali metal oxide (R.sub.2 O) selected from 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O and 10-30% Cs.sub.2 O. These oxides total at least 85% of the glass composition. Copper and/or cadmium oxides, together with sufficient chlorine, bromine and/or iodine to react stoichiometrically with at least 0.3% copper and/or cadmium, are added to the base glass composition.
The disclosed copper and/or cadmium halide glasses are described as having certain advantages with respect to the better known silver photochromic glasses. In particular, they are described as having an optical density more closely related to incident actinic radiation, and also as being less temperature dependent.
U.S. Pat. No. 5,281,562 (Araujo et al.) is directed to a family of glasses which also contain a cuprous and/or cadmium halide crystal phase, and which are strongly absorbing in the ultraviolet region of the radiation spectrum. They have a sharp spectral cut-off of shorter wavelengths at about 400 nm. However, they are non-photochromic, and may be essentially colorless. As expressed in cation %, the compositions of these glasses consist essentially of 35-73% SiO.sub.2, 15-45% B.sub.2 O.sub.3, 0-12% Al.sub.2 O.sub.3, the content of Al.sub.2 O.sub.3 being less than 10% when the SiO.sub.2 content is greater than 55%, 4.75-20% Li.sub.2 O+Na.sub.2 O+K.sub.2 O consisting of 0-12% Li.sub.2 O, 0-20% Na.sub.2 O and 0-12% K.sub.2 O, 0-5% CaO+SrO+BaO consisting of 0-5% CaO, 0-5% SrO, and 0-5% BaO, 0.125-1% Cu.sub.2 O, 0-1% CdO, 0-5% ZrO.sub.2, 0-0.75% SnO.sub.2, 0-1% As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3, the glass also containing in weight %, 0.25-2% Cl+Br consisting of 0-1.75% Cl and 0-1% Br, and 0-2% F, and having an R-value, calculated in cation %, of 0.15-0.45, the R-value not exceeding 0.30, except as the glass composition meets at least one of the following conditions: up to 12 cation % Li.sub.2 O, less than 10 cation % Al.sub.2 O.sub.3, at least 0.3 cation % Cu.sub.2 O, 0.5-2 weight percent Cl+Br.
The application explains that cadmium is not required to secure absorption of ultraviolet radiation, and imparts no exceptional benefit to the glass. The halide crystals are effective to absorb ultraviolet radiation, but cadmium is known to be extremely toxic. Accordingly, its presence is severely restricted, and it is preferably completely avoided. Therefore, while cadmium halide glasses are within the scope of the present invention, reference throughout will be to copper halide glasses.
U.S. Pat. No. 3,540,793 (Araujo et al.) discloses that photochromic glasses, based on silver halide crystals, can also be rendered polarizing in the darkened state. This dual effect is accomplished by thermally softening and stretching the softened glass. This elongates the silver halide crystals and generates a birefringent effect in the glass. When the glass is darkened, the anisotropy of the silver halide particles is also manifested in an observed dichroism.
Subsequently, U.S. Pat. No. 4,304,584 (Borrelli et al.) reports that a permanent polarizing effect can be obtained. This is accomplished by heat treating the stretched glass, containing the elongated silver halide crystals, in a reducing atmosphere. This thermal reduction treatment is carried out at a temperature below the glass annealing point, but above 300.degree. C. It chemically reduces the elongated silver halide crystals to elongated particles of metallic silver. Still later, it was learned that the polarizing effect, obtained by thermal reduction, could be extended to the infra-red portion of the spectrum. This is reported in U.S. Pat. No. 4,479,819 (Borrelli et al.).
It has been found that the copper and cadmium halide photochromic glasses disclosed in U.S. Pat. No. 3,325,299 can also be rendered polarizing, when in the darkened state, by thermally softening the glass and stretching, or otherwise elongating, it. This action elongates the halide crystals and is described in detail in U.S. Pat. No. 3,954,485 (Seward III, et al.).
The present invention provides glasses containing copper halide crystals and having unique polarizing characteristics. It further provides such glasses having the property of a sharp spectral cut-off at about 400 nm. It also provides a method of imparting the unique polarizing characteristics to known glasses containing copper halide crystals.